It may or may not be an error depending what you are trying to do. It's rather safe that the system avoid duplicates entries in that tables.
Want to replace data ? Delete the row prior the import.
Otherwise, you may set IGNORE_DUPS, it will ignore the duplicate rows to be loaded.

Datamover tries to insert data in PSOPRDEFN and rows exist with these keys.

To avoid this you can add the following statement in your script:
SET UPDATE_DUPS;

This will update the values other than the keys if a row already exists in the database with the keys in the dat file.

I can advice you to buy the new book of Jim Marion, PeopleSoft PeopleTools Data Management and Upgrade Handbook
http://www.amazon.com/PeopleSoft-PeopleTools-Management-Upgrade-Handbook/dp/0071787925/ref=sr_1_1?ie=UTF8&qid=1353575734&sr=8-1&keywords=jim+j+marion